1. Technical Field
This invention relates to microwave solid state devices, and more particularly to high power microwave pseudomorphic gallium arsenide high electron mobility transistors.
2. Discussion
Microwave semiconductor devices are used for the generation, amplification, detection, and control of electromagnetic energy in the frequency range of about 1-100 GHz. At such high frequencies, the usefulness of conventional transistors is severely limited by high-frequency effects, which result in a much lower signal-to-noise ratio.
Improvements in microwave field effect transistors have been realized by the use of gallium arsenide rather than silicon because the mobility of electrons of gallium arsenide is several times as high as in silicon. This higher mobility is particularly important at microwave frequencies, since it leads to lower series resistance and larger device dimensions for devices designed for operation at a given frequency.
For both low noise and power applications, pseudomorphic (PM) InGaAs high electron mobility transistors (HEMTs) have demonstrated superior millimeter wave performance. The large conduction-band discontinuity at the AlGaAs/InGaAs heterointerface allows for a high two-dimensional electron gas concentration. The InGaAs channel also offers superior electron transport properties (mobility and saturation velocity) compared to conventional GaAs channels used in AlGaAs/GaAs HEMTs.
However, previously reported power performance of PM InGaAs HEMT's have yielded relatively low gain. For example, in one report, after accounting for 2-dB insertion loss, the gain at power saturation was only 2-3 dB. See P. M. Smith et al. "A 0.25-micro gate-length pseudomorphic InGaAs HE FT with 32-mw output power at 94 GHZ," IEEE Electron Device Lett. Vol. 10, No. 10, pp. 437-439, 1989, which is herein incorporated by reference. Also, the peak transconductance (gm) v. gate-to-source voltage (Vgs) curve for such devices is relatively narrow.
Thus, it would be desirable to have a microwave power transistor which can operate at about 94 GHz and produce improved gain and a broad gm v. Vgs curve at high power levels.